Production of nitrophosphate fertilizer,ammonium nitrate,and calcium sulfate or calcium carbonate

ABSTRACT

NITROPHOSPHATE FERTILIZER IS PRODUCED BY THE NITRIC ACID DIGESTION OF PHOSPHATE ROCK. THE PH OF THE INITIAL DIGESTION SOLUTION IS RAISED TO THE RANGE OF ABOUT 1.8 TO 2.5 BY AMMONIA ADDITION, TO PRECIPITATE IMPURITIES INCLUDING A MAJOR PORTION OF FLUORIDE AND IRON PHOSPHATE. THE SOLID PRECIPITATE IS FILTERED FROM THE CLEAR SOLUTION, AND THE SOLID FILTER CAKE IS CALCINED AT ELEVATED TEMPERATURE TO VOLTALIZE FLUORIDE AND DECOMPOSE IRON PHOSPHATE INTO A NITRIC ACID-INSOLUBLE FORM. THE CALCINED CAKE IS DIGESTED WITH NITRIC ACID TO RECOVER PHOSPHATE AND ALUMINUM VALUES IN SOLUTION, AND THE RESULTING SLURRY IS FILTERED AND THE SOLID FILTER CAKE CONTAINING IRON COMPOUNDS AND SILCA IS DISCARDED. THE CLEAR FILTRATE IS COMBINED WITH THE INITIAL CLEAR SOLUTION PREVIOUSLY FORMED BY FILTRATION TO REMOVE SOLID FLUORIDE AND IRON IMPURITIES, AND THE COMBINED SOLUTION IS PREFERABLY FURTHER ACIDIFIED TO REDUCE THE PH TO A LOW AND HIGHLY ACID VALUE. A SOLUBLE SALT SUCH AS AMMONIUM SULFATE OR POTASSIUM SULFATE, WHICH HAS AN ANION WHICH FORMS AN INSOLUBLE SOLID SALT WITH THE CALCIUM CATION, IS ADDED TO THE COMBINED SOLUTION, AND AN INSOLUBLE SOLID CALCIUM SALT IS PRECIPITATED. FILTRATION REMOVES THE SOLID CALCIUM SALT, AND AMMONIA IS ADDED TO THE REMAINING SOLUTION TO RAISE THE PH ABOVE 3.0 AND PRECIPITATE REMAINING IMPURITIES. AFTER REMOVAL OF PRECIPITATED SOLIDE IMPURITIES BY FILTRATION, THE FINAL SOLUTION IS PROCESSED BY WATER REMOVAL TO PRODUCE SOLID NITROPHOS FERTILIZER, USUALLY TOGETHER WITH AMMONIUM NITRATE AS A CO-PRODUCT.

June 1, 1971 s, STRELZOFF ETAL 3,582,313

PRODUCTION OF NITROPHOSPHATE FERTILIZER AMMONIUM NITRA'IE, AND

- CALCIUM SULFATE on CALCIUM CARBONATE Filed Oct. 9, 1968 SAMUELSTRELZOFF SYDNEY ATKIN INVENTORS.

"AGENT Patented June 1, 1971 3,582,313 PRODUCTION OF NITROPHOSPHATEFERTIL- IZER, AMMONIUM NITRATE, AND CALCIUM SULFATE R CALCIUM CARBONATESamuel Strelzolf, New York, N.Y., and Sydney Atkin, Springfield, N.J.,assignors to Chemical Construction Corporation, New York, N.

Filed Oct. 9, 1968, Ser. No. 766,139 Int. Cl. C05b 11/06 U.S. CI. 7135 8Claims ABSTRACT OF THE DISCLOSURE Nitrophosphate fertilizer is producedby the nitric acid digestion of phosphate rock. The pH of the initialdigestion solution is raised to the range of about 1.8 to 2.5 by ammoniaaddition, to precipitate impurities including a major portion offluoride and iron phosphate. The solid precipiitate is filtered from theclear solution, and the solid filter cake is calcined at elevatedtemperature to volatilize fluoride and decompose iron phosphate into anitric acid-insoluble form. The calcined cake is digested with nitricacid to recover phosphate and aluminum values in solution, and theresulting slurry is filtered and the solid filter cake containing ironcompounds and silica is discarded. The clear filtrate is combined withthe initial clear solution previously formed by filtration to removesolid fluoride and iron impurities, and the combined solution ,ispreferably further acidified to reduce the pH to a low and highly acidvalue. A soluble salt such as ammonium sulfate or potassium sulfate,which has an anion which forms an insoluble solid salt with the calciumcation, is added to the combined solution, and an insoluble solidcalcium salt is precipitated. Filtration removes the solid calcium salt,and ammonia is added to the remaining solution to raise the pH above 3.0and precipitate remaining impurities. After removal of precipitatedsolid impurities by filtration, the final solution is processed by waterremoval to produce solid nitrophos fertilizer, usually together withammonium nitrate as a co-product.

BACKGROUND OF THE INVENTION Field of the invention The invention relatesto the digestion of phosphate rock, shale or the like with nitric acid,and the processing of the resulting solution with ammonia and a sulfatesalt or the like, to respectively precipitate impurities and solidcalcium sulfate, so as to produce a final purified solution from which asolid nitrophosphate fertilizer containing a high proportion ofwater-soluble phosphate is produced by water removal. The term phosphaterock as employed herein will be understood to encompass the varioustypes of phosphate shale and other phosphatic materials of similarcompositions, as well as phosphate rock per se.

Description of prior art The conventional procedure for the processingof phosphate rock to produce soluble phosphates for usage as 'therebybecome available to plants when applied to the soil. In some instances,a more highly concentrated product is produced, by reacting thephosphate rock with aqueous sulfuric acid to produce a slurry of solidcalcium sulfate in phosphoric acid solution. The solid calcium sulfateis removed by filtration, and the phosphoric acid solution isconcentrated and reacted with further phosphate rock to produce triplesuperphosphate fertilizer.

The acidulation or digestion of phosphate rock with nitric acid has beendescribed in US. Pat. No. 2,689,175 and U.S. patent application No.680,757 filed Nov. 6, 1967, and now abandoned. In the patented process,the ground phosphate rock is reacted with aqueous nitric acid, and thesulfate radical required for the elimination of calcium via theformation of solid calcium sulfate is supplied by adding an ammoniumsulfate solution to the reaction mixture. These reagents are combined inthe proper proportion to produce a slurry of filterable calcium sulfate,or gypsum, in an aqueous solution of phosphoric acid and ammoniumnitrate. The slurry is filtered, and the clear solution is ammoniatedand concentrated in stages, to principally produce ammonium phosphateand ammonium nitrate. French Pat. No. 705,887 provides a similarprocedure, and descriptions of prior commercial techniques are given inChemical Engineering, v. 74, No. 22, dated Oct. 23, 1967, at pages86-88, 90, 134-136. The recovery of valuable byproduct such as aluminumand uranium values from phosphatic material, specifically leached zonephosphates, is described in US. Pat. No. 3,126,249.

SUMMARY OF THE INVENTION In the present invention, a nitrophosphateproduct having high availability and water-solubility of phosphatevalues is produced, by the nitric acid acidulation of phosphate rock andsubsequent ammoniation in an improved process sequence which featuresammoniation of the initial digestion solution to precipitate impuritiessuch as fluorine and iron, calcining of the precipitate to volatilizefluorine and convert iron to an acid-insoluble state, and nitric aciddigestion of the calcined precipitate to solubilize and recoverphosphate and aluminum values.

The feed phosphate rock, which may be initially ground to a powder orthe like to aid in reaction, is digested with a first nitric acid streamto form a solution containing dissolved phosphate. Ammonia is added tothe solution to raise the pH of the solution to the range of about 1.8to 2.5, which causes the formation .of a solid precipitate containingmost of the fluoride and iron phosphate impurities. The solidprecipitate is separated from the clear solution and calcined at anelevated temperature generally in the range of 500 C. to 1000 C. tovolatilize fluorine-containing iinpurities and convert iron phosphate toacid-insoluble compounds by thermal decomposition. The calcinedprecipitate is digested with a second nitric acid stream, to solubilizeand dissolve phosphate and aluminum values. Residual solid material fromthis second digestion step, consisting mostly of insoluble ironcompounds and silica, is discarded. The clear liquid solution from thedigestion of the calcined precipitate is added to the initial clearsolution, and the pH of the combined clear solution may be lowered below1.0 by the addition of further nitric acid. In any case, a soluble saltsuch as ammonium sulfate or potassium sulfate, which has an anion whichforms an insoluble solid salt with the calcium cation, is added to thesolution; so that calcium nitrate in the combined solution is reacted toform a soluble nitrate and an insoluble solid calcium salt, such ascalcium sulfate, which is removed from the solution. Ammonia is thenadded to the clear resulting solution, to raise the solution pH to therange of about 3.0 to about 4.5 and form a solid precipitate containingimpurities which is filtered from the solution. The final clear solutionprincipally contains dissolved ammonium phosphate, mostly present asmono-ammonium acid phosphate, together with ammonium nitrate, and isprocessed by water removal to produce a nitrophos product. The nitrophosis generally produced by partial vacuum evaporation and crystallization,to produce a solid nitrophos precipitate in an aqueous ammonium nitratemother liquor, which is separated from the solid nitrophos and furtherconcentrated to produce a substantially anhydrous ammonium nitrate meltwhich is prilled or otherwise processed to form discrete particles ofsolid ammonium nitrate fertilizer.

The process of the present invention provides several salient novelfeatures and advantages. The calcining step is highly important, becausethe calcining of the first ammoniation precipitate serves to volatilizemore than 80% of the fluorine and decompose the iron phosphate, thusmaking the iron insoluble in the following redigestion step. The processpermits the recovery of aluminum, uranium and rare earth values from thesecond ammoniation precipitate, typically by such procedures asdescribed in US. Pat. No. 3,126,249. In this respect, after recovery bybyproduct metals values, the residual solution can be recycled to thepresent process to recover phosphate values. A relatively pure calciumsulfate or gypsum byproduct can be produced, which is suitable for usagein the manufacture of building materials or compositions or the like. Ithas been determined that the filtration rates of the various slurriesand precipitates in'the process of the present invention are improved,as compared to prior art procedures, principally because the calciningstep destroys the gelatinous slimes or the like which are formed orpresent in conventional processes for acid digestion of phosphate rock.A nitrophosphate product is produced in which more than 90% of the totalphosphate is water soluble. In this respect, the removal of impuritiessuch as fluorine, iron and calcium helps to prevent the reversion ofphosphate in the final nitrophosphate product to the citrate-insolublestate. As will appear infra, a feed-grade dicalcium phosphate co-productmay also be readily produced in conjunction with the present inventionby usage of a portion of one of the process streams in accordance withthe procedure of US. patent application No. 680,757 filed Nov. 6, 1967,and now abandoned. Finally, the coproduct ammonium nitrate forms asuperior prilled product when the mother liquor solution is concentratedto an anhydrous melt and prilled, because the presence of small amountsof ammonium sulfate and ammonium phosphate in the final solid prillsproduces an ammonium nitrate prill structure and composition whichresists disintegration at a transition point.

It is an object of the present invention to provide an improved processfor the production of nitrophosphate fertilizer by the nitric aciddigestion of phosphate rock.

Another object is to remove fluoride and iron impurities in an improvedmanner during the processing of phosphate rock with nitric acid.

A further object is to improve filtration rates of slurries during theprocessing of phosphate rock with nitric acid, in which precipitationfrom solutions is obtained by ammoniation or by the addition of asoluble salt having an anion which forms an insoluble solid salt withthe calcium cation.

An additional object is to produce an nitrophos product in which most ofthe phosphate is in a water-soluble form, by the nitric acid acidulationof phosphate rock and subsequent ammoniation in two stages.

An object is to recover aluminum, uranium and rare earths values fromphosphate rock in a concentrated byproduct precipitate, during thenitric acid digestion of phosphate rock and subsequent ammoniation.

Still another object is to produce relatively pure byproduct gypsumduring the nitric acid digestion of phosphate rock to producenitrophosphate fertilizer.

Still a further object is to produce an improved form of ammoniumnitrate prill composition during the nitric acid digestion of phosphaterock followed by ammoniation and separation of solid nitrophosphate fromresidual aqueous ammonium nitrate mother liquor.

Finally, an object is to provide a process for the production ofnitrophos by the nitric acid digestion of phosphate rock in which feedgrade dicalcium phosphate may be produced as a co-product of the processby selective ammoniation of a portion of an intermediate purifiedprocess stream.

These and other objects and advantages of the present invention willbecome evident from the description which follows.

DESCRIPTION OF THE DRAWING AND PREFERRED EMBODIMENTS Referring now tothe drawing, a flowsheet of a preferred embodiment of the invention ispresented. Ground phosphate rock stream 1 and aqueous nitric acid stream2 are passed into digester 3, and the phosphate rock is digested andsolubilized in unit 3. In addition to the reaction of tricalciumphosphate with nitric acid, small amounts of calcium carbonate andcalcium fluoride present in stream 1 will react with stream 2 to yieldcalcium nitrate and volatile oif-gases such as carbon dioxide andhydrogen fluoride. A portion of the hydrogen fluoride reacts and remainsin the liquid phase as combined fluorine, either as a fluoride orsilicofluoride. The phosphate rock stream 1 will generally analyze inthe range of about 25% to 38% or higher equivalent phosphorus pentoxidecontent, about 45% to 50% equivalent calcium oxide content, and about2.5% to 4% equivalent fluorine content, all in percentages by weight.The aqueous nitric acid stream 2 will generally contain in the range ofabout 20% to 70% nitric acid content by weight, and sufficient flow rateof stream 2 relative to stream 1 is provided to maintain a pH in therange of about 0.3 to 1.5 in unit 3. A temperature level generally inthe range of about 55 C. to C. is maintained within digester 3, andagitator or stirrer 4 is provided to circulate the reactants within unit3 and produce a uniform solution, which may contain a small amount ofslimy sludge insoluble in the acid solution. As mentioned supra, theslimy sludge is entrained in the initial precipitate and is eliminatedin the calcining step, as will appear infra.

The aqueous solution 5 discharged from unit 3 principally containscalcium nitrate and an acid salt of calcium phosphate, generallymonocalcium phosphate. Stream 5 is passed into ammoniation vessel 6together with anhydrous or aqueous ammonia stream 7, which serves toraise the pH of the solution to a level generally in the range of about1.8 to about 2.5, and to form a solid precipitate containing impuritiessuch as fluorides and iron phosphate, together with the undigested slimysludge. The resulting slurry stream 8 discharged from unit 6 is passedto solids filter 9, from which a solid filter cake stream 10 s removedand processed in accordance with the present mvention.

Stream 10 contains calcium phosphate values, together with fluoride andiron phosphate impurities, a small proportion of aluminum values and theslimy residual sludge from digestion in unit 3. Stream 10 is passed intocalciner 11, which generally consists of a rotary kiln or the like. Fuelstream 12, which usually consists of a fluid hydrocarbon fuel such asresidual oil, fuel oil or natural gas, is passed into kiln 11, togetherwith combustion air stream 13, and the combustion of stream 12 withinunit 11 serves to generate a highly elevated temperature within kiln 11,generally in the range of about 500 C. to 1000 C. Solids filter cakestream 10 is calcined within unit 11, with resultant volatilization ofmost of the contained fluorine or fluoride compounds, which aredischarged from fliln 11 in flue gas stream 14. In this man ner, thedischarge of stream 14 serves to eliminate a major portion of thefluorine from the subsequent processing steps and products. In addition,iron phosphate contained in the solids feed is converted to insolubleiron compounds in unit 11.

The resultant calcined filter cake solids stream 15 discharged from unit11 is passed into digester 16 together with aqueous nitric acid stream17. Unit 16 is similar in configuration and function to unit 3 describedsupra, and Serves to digest 'and 'solubilizea major portion ofstream-15, thus most of the phosphate and aluminum values instream 15are dissolved into solution in unit 16 while iron compounds and silicaremain in a solid undissolved state. A temperature level generally inthe rangeof about 60 C. to 90 C. is maintained in unit 16. The resultingslurry'stream 18 discharged from unit 16 is passed to solids filter 19,from which a solid filter cake stream 20 is removed and discharged towaste. Stream 20 principally contains silica and iron compounds derivedfrom stream 15 and the discharge of stream 20 thus serves to eliminate amajorportion of the iron from the subsequent processing steps andproducts.

A clear solution stream 21 containing dissolved phosphate and aluminumvalues is removed from filter unit 19 and passed to solution combiningand acidification -vessel 22. Returning to filter unit 9, a clearaqueous solution stream 23 containing dissolved calcium nitrate,ammonium nitrate and phosphate values in the form of phosphoric acid andmonoammonium acid phosphate is removed from unit 9. Stream 2.3 ispreferably divided into stream 24, which is processed for the recoveryof feed grade dicalcium phosphate in a manner to be described infra, andstream 25, which is passed to solution combining vessel 22 forcombination with stream 21. The combined liquid phase in vessel 22 willgenerally have a pH below 2.5, and the pH of the combined solution ispreferably adjusted to a value below 1.0 and preferably in the range of0.3 to 0.7 by adding concentrated aqueous nitric acid stream 26 tovessel 22. Stream 26 will generally contain in the range of about 35% to70% nitric acid content by Weight.

The resulting acidified solution stream 27 removed from vessel 22 is nowpassed to conversion vessel 28, in

,which calcium ions are precipitated as a solid calcium salt by theaddition of a soluble salt to the solution,

which soluble salthas an anion which forms an insoluble solid salt withthe calcium cation in the solution. The preferred soluble salt added.tovessel 28 via stream 29 is ammonium sulfate or potassium sulfate,however other soluble sulfates, carbonates such as ammonium carbonate,"onthe' like may" be employed as stream 29" within the scope ofthepresent'invention: -In this preferred embodiment of the invention,stream 29 consists of an aqueous ammonium sulfate solution. Calcium ionsare precipitated in unit 28 as insoluble calcium sulfate, and

a slurry of precipitated solid calcium sulfate in an aqueous solutionphasecontaining ammonium nitrate, mono-ammonium phosphate and phosphoricacid is produced in ,unit 28. The pH of the slurry in unit 2 8 ismaintained in the range of about 0.5 to 2.0, and the temperature of theslurry is generally in the range of about 50 C. to 100 C.Theresultingslurry stream'30 discharged from unit 28 is passed to solidsfilter 31, from which a solid filter cake stream 32 is removed. Stream32 consists of relatively pure calcium sulfate or gypsum, and may bedischarged to waste or employed in the manufacture of building materialsor compositions or the like. The re-v moval'of stream 32 serves toeliminate most of the calcium from' the subsequent processing steps andand passed to ammoniation vessel 34, to which ammonia is added viastream 35 which raises the solution pH to] free phosphoric acid in theprocess solution, to form further monoammonium phosphate. A minorproportion of diammonium phosphate may form a solution at higherdischarged from unit 34 is passed to solids filter 37, from 'pH values.In any case, the resulting slurry stream 36'- which a solids filter cakestream 38 is discharged. Solids stream 38 contains a minor amount ofphosphate values, together with concentrated proportions of impuritiesincluding aluminum, uranium and rare earths values. Stream 38 may beprocessed in accordance with the procedure of US. Patent No. 3,126,249,to recover aluminum and uranium values.

A clear aqueous solution stream 39 is discharged from unit 37. Stream39-is now relatively free of impurities such as fluorine, iron andcalcium, and consists primarily of an aqueous solution containingdissolved ammonium nitrate and monoammonium acid phosphate. Because ofthe elimination of impurities as described supra, the subsequent solidnitrophos fertilizer product produced from stream 39 by water removal,as will appear infra, is relatively pure and reversion of phosphatevalues to a citrateinsoluble form is substantially prevented. Stream 39is now passed to evaporator 40, which in most cases consists of a vacuumcrystallizerwhich is heated by steam coil 41. Water vapor is removedfrom unit 40 via stream 42, which generally extends to a source ofvacuum such as a steam jet exhauster. The removal of water in unit40'causes the selective precipitation of nitro-phosphate crystalsconsisting primarily of monoammonium phosphate, in an aqueous motherliquor solution principally containing dissolved ammonium nitrate. Theresulting slurry stream 4 3 discharged from unit 40'is passed to solidsIfilter 44, from which solid filter cake stream 45 consisting of productnitrophos fertilizer principally containing monoammonium "phosphate ispassed to product utilization.

A clear mother liquor stream 46 is also removed from filter 44. Stream46 consists essentially of an'aqueous ammonium nitrate solution, andstream 46 is now preferably passed to evaporator 47 for heating viasteam coils 48 to produce a substantially anhydrous ammonium nitratemelt for prilling.

Returning to solids filter 9, a portion of the discharge clear solutionstream 23 is preferably utilized via stream 24 for the production offeed grade dicalcium phosphate co-product. In this case, sufficientammonia will be added to unit 6 via stream 7 to produce a somewhathigher pH in the 'range of about 2.1 to 2.5 in unit 6, so as to attainstream 23, and is an aqueous solution containing dissolved calciumnitrate, ammonium nitrate and phosphate values in the form of phosphoricacid and monoammonium acid phosphate, inequilibrium with monocalciumphosphate. Stream 24, is passed into neutralizer vessel 49 together withammonia stream 50, which serves to raise the solution pH to a levelgenerally in the range of about 3.0 to 6.0, and to precipitate solid andrelatively pure crystalline dicalcium phosphate in aqueous solution. Theresulting slurry stream 51 discharged from unit 49 is passed to solidsfilter 52, from which solid filter cake stream 53 consistiirg ofsubstantially pure or feed grade dicalcium phosphate is removed andpassed to product utilization.

A- clear solution stream 54 containing dissolved calcium nitrate andammonium nitrate is discharged from unit 52, and passed to conversionvessel 55, in which calcium ions are precipitated as solid calciumcarbonate by the in situ formation of ammonium carbonate in solution.Ammonia stream 56 and carbon dioxide stream 57 are passed into unit 55,and essentially form ammonium carbonate in the range of about 30 to 45and precipitates an aqueous solution, which reacts with dissolvedcalcium ni- -trate to form SOlld calcium carbonate and further dissolvedammonium nitrate. The slurry stream 58 discharged from unit 55 is passedinto solids filter 59, from which solid calcium carbonate filter cakestream 60 is discharged to waste disposal.

Theiclear solution stream 61 discharged from unit 59 consists of anaqueous ammonium nitrate solution, which is passed to concentrator 47together with stream 46 as described supra. The combined aqueousammonium nitrate solution in unit 47 is evaporated to produce aconcentrated solution or substantially anhydrous melt, by circulatingsteam or other heating medium through coil 48. Evaporated water vapor isdischarged from unit 47 via stream 62. Unit 47 may be of an alternativefalling film evaporator configuration in practice, in which case streams46 and 61 would flow downwards as a liquid film on the inner surfaces ofvertical tubes, through which a heated air stream is circulated, withsteam heating external to the tubes. In any case, a substantiallyanhydrous ammonium nitrate melt stream 63 is produced by unit 47, whichmay be of any suitable configuration to accomplish this result. Stream63 is now sprayed or otherwise dispersed into the top of prilling tower64, which is of conventional configuration and is usuallyair-ventilated. Solid prills of ammonium nitrate are formed in unit 64,and the resulting product stream 65 consisting of ammonium nitrateprills is passed to product utilization, such as in fertilizer usage. Asmentioned supra, the solid prills of stream 65 are of improved form andconfiguration, due to the presence of small amounts of solid ammoniumsulfate and ammonium phosphate in the prill structure, which provide animproved form of prill structure which resists disintegration at thetransition point.

Numerous alternatives within the scope of the present invention, besidesthose mentioned supra, will occur to those skilled in the art. Theranges of process variables enumerated supra, such as solutionconcentrations and pH and temperature ranges, constitute preferredembodiments for optimum utilization of the process concepts of thepresent invention, and the invention may be practices outside of theseranges in suitable instances, with the exception of the pH of thesolutions in units 6 and 34 formed by the respective ammoniation steps.It will be evident that the various reaction, digestion and solidsprecipitation vessels may be provided with suitable mixing and agitationdevices in practice, such as element 4. The precipitation of a solidcalcium salt from process stream 54, with the concomitant formation offurther ammonium nitrate in solution, may be attained in practice byreacting the dissolved calcium nitrate with any suitable ammonium salthaving example, unit 11 could consist of a vertically oriented fluid bedroaster, with powdered coal or other suitable fuel such as a fluidhydrocarbon fuel being directly injected into the fiuid bed or added tostream 10. In some cases, the ancillary production of feed gradedicalciurn phosphate as a coproduct may not be desired or warranted bymarket conditions, in which case stream 24 would be eliminated, with allof stream 23 passing to further processing via stream 25 and units 49,-52, 55 and 59 being Omitted from the process sequence. In some instancesstreams 46 and 61, or stream 63, may be directly passed to productutilization as solution-type fertilized products of high ammoniumnitrate content. If liquid stream 63 is a product of the process, itwill be evident that unit 47 will be operated so as to produce stream 63as a concentrated aqueous ammonium nitrate solution, rather than as ananhydrous melt.

An example of a batch-wise laboratory scale investiga tion of theprocess of the present invention will now be described.

EXAMPLE Ground Florida phosphate rock was employed in the laboratoryscale investigation of the precess, and the optional ancillaryproduction of feed grade dicalcium' phosphate was omitted from thelaboratory study. The compositions of the various principal processstreams were anlyzed and the analysis was reported in terms ofequivalent weight percent of the respective component expressed as asuitable analytical product, thus phosphorus content was expressed asequivalent phosphorus pentoxide content by weight. In addition, solutionconcenrtations are expressed as percent by weight of the respectivecomponent in aqueous solution. Following are the test results of thelaboratory-scale practice of the process. Table I presents data relativeto the principal process streams bearing significant phosphate values,and Table I1 provides data relative to the balance of the processstreams employed or obtained in the batch-type laboratory investigation.Table III presents data relative to conditions maintained in theprincipal process units described supra.

TABLE I.PRINCIPAL SOLID PHOSPHATE-BEARING STREAMS Stream Noumber ananion which forms an insoluble salt with the calcium Mass gram, cation,such as ammonium sulfate. Some or all of the Annli siawt'ffirfiii solidsfilters 9, 19, 31, 37, 44, 52 and 59 may in practice 3-8 consist of anysuitable device or apparatus for separating a solid phase from a liquidphase, such as a centrifugal j 3%; separator, a vacuum drum filter, aplate and frame filter 0:08 press or the like. The calciner 11preferably consists of a 0. 687 rotary kiln, however unit 11 mayalternatively consist of 33 a fluid bed roaster or calciner or the like,which may be 8:70 heated by any suitable heating means or elements. For

Stream No 2 7 17 20 26 '29 a2 a5 4s *2? :33 Analysis, wt. percent TotlPONHa- (NHOISO 1 Volume in milliliters.

I Digestion period duration was 45 minutes.

In summary, it will be evident that the nitrophos product obtained fromunit 44 as stream 45 possesses highly valuable fertilizercharacteristics, specifically with respect to total phosphate contentand phosphate solubility, as

evidenced by the analysis data for stream 45 in Table I supra. Inaddition, at least 81% of the input phosphate values is recovered in thefinal nitrophosphate fertilizer product. Further phosphate values may berecovered from stream 38, as well as alumina, uranium and rare earthvalues, by processing such as disclosed in US. Pat. No. 3,126,249.

What is claimed is:

1. A process for the production of nitrophosphate fertilizer by thenitric acid acidulation of phosphate rock which comprises (a) digestingphosphate rock with a first nitric acid stream, whereby a solution isformed at a pH in the range of about 0.3 to 1.5 and a temperature in therange of about 55 C. and 80 C., said solution containing dissolvedphosphate,

(b) adding ammonia to said solution, whereby the pH of said solution israised to the range of about 1.8 to about 2.5, and a major portion ofthe fluoride and iron phosphate in said solution are precipitated,

(c) filtering said solution to remove precipitated solids, whereby asolid filter case is separated from residual clear solution,

(d) calcining said solid filter cake at elevated temperature in therange of about 500 C. to 1000 C., whereby contained fluorine isvolatilized and iron phosphate is decomposed to form nitricacid-insoluble iron compounds,

(e) digesting the calcined filter cake with a second nitric acid streamat a temperature in the range of about 60 C. to 90 C., whereby a slurryis formed containing phosphate and aluminum values in the liquid phase,

(f) filtering said slurry, whereby a solid phase containing ironcompounds and silica is removed from the clear liquid phase,

(g) combining the clear liquid phase from step (f) with said residualclear solution of step (c), to form a combined solution at a pH below2.5, and adding nitric acid to the combined solution to effect a pHbelow 1.0,

(h) adding a soluble salt to said combined solution,

said soluble salt being selected from the group consisting of solublesulfate and soluble carbonates which form an insoluble solid salt withthe calcium cation in said combined solution, whereby calcium nitrate insaid combined solution is reacted to form a soluble nitrate and aninsoluble solid calcium salt,

(i) filtering the resulting solution, whereby a solid phase principallyconsisting of a solid calcium salt is removed from the clear resultingsolution,

(j) adding ammonia to said clear resulting solution, whereby the pH ofsaid solution is raised to the range of about 3.0 to about 4.5, and asolid precipitate containing impurities forms in said resultingsolution,

(k) filtering said solid precipitate from said resulting solution toproduce a final solution containing dissolved ammonium phosphate andammonium nitrate, and

(l) removing water from said final solution, to produce a productnitrophos fertilizer containing ammonium phosphate.

2. The process of claim 1, in which water is removed from said finalsolution according to step (1), and product nitrophos fertilizer isproduced, by evaporating water from said solution to produce solidnitrophos crystals in an aqueous mother liquor solution principallycontaining dissolved ammonium nitrate, separating said solid nitrophoscrystals from said aqueous mother liquor solution, and recoveringproduct ammonium nitrate from said aqueous mother liquor solution.

3. The process of claim 2., in which product ammonium nitrate isrecovered from said aqueous mother liquor solution by evaporating waterfrom said mother liquor solution to produce a substantially anhydrousmelt principally containing ammonium nitrate together with minor amountsof impurities including ammonium. phosphate and ammonium sulfate, andprilling said melt to produce a solid prills product principallycontaining ammonium nitrate.

4. A process for the production of nitrophosphate fertilizer by thenitric acid acidulation of phosphate rock which comprises (a) digestingphosphate rock with a first nitric acid stream, whereby a solution isformed at a pH in the range of about 0.3 to 1.5 and a temperature in therange of about 55 C. to C., said solution containing dissolvedphosphate,

(b) adding ammonia to the solution formed according to step (a), wherebythe pH of said solution is raised to the range of about 1.8 to about2.5, and a major portion of the fluoride and iron phosphate contained insaid solution are precipitated from said solution,

-(c) filtering the solution produced by step (b), whereby a solid filtercake is separated from a residual clear solution,

(d) calcining the solid filter cake produced by step (c) at atemperature in the range of 500 C. to '1000 C., whereby a major portionof the contained fluorine is volatilized and iron phosphate isdecomposed to form iron compounds insoluble in nitric acid,

(e) digesting the calcined filter cake with a second nitric acid streamat a temperature in the range of about 60 C. to 0., whereby a slurry isformed, said slurry containing phosphate and aluminum values from saidcalcined cake in the liquid phase,

(f) filtering said slurry, whereby a solid phase containing ironcompounds and silica is removed from the liquid phase containingdissolved phosphate,

(g) combining the liquid phase from step (f) with said residual clearsolution formed by step (c),

(h) adding nitric acid to the combined liquid solution formed by step(g), whereby the pH of said solution is lowered to the range of about0.3 to 0.7,

(i) adding a soluble sulfate salt selected from the group consisting ofammonium sulfate and potassium sulfate to the acidified solution of step(h), whereby solid calcium sulfate is precipitated from said solutionand a soluble nitrate is formed in said solution,

(j) filtering the resulting solution to remove precipitated solidcalcium sulfate and produce a clear resulting solution, I

(k) adding ammonia to said clear resulting solution whereby the pH ofsaid solution is raised to the range of about 3.0 to about 4.5, and asolid precipitate containing impurities forms in said resultingsolution,

(1) filtering said solid precipitate from said resulting solution toproduce a final solution containing dissolved ammonium phosphate andammonium nitrate, and

(m) removing water from said final solution, to produce productnitrophos fertilizer containing ammonium phosphate.

1 1 5. The process of claim 4, in which said soluble salt addedaccording to step (i) is ammonium sulfate.

6. The process of claim 4, in which said soluble salt added according tostep (i) is potassium sulfate.

7. The process of claim 4, in which water is removed from said finalsolution according to step (m), and product nitrophos fertilizer isproduced, by evaporating water from said solution to produce solidnitrophos crystals in an aqueous mother liquor solution principallycontaining dissolved ammonium nitrate, separating said solid nitrophoscrystals from said aqueous mother liquor solution, and recoveringproduct ammonium nitrate from said aqueous mother liquor solution.

8. The process of claim 7, in which product ammonium nitrate isrecovered from said aqueous mother liquor solution by evaporating waterfrom said mother liquor solution to produce a substantially anhydrousmelt principally containing ammonium nitrate together with minor UNITEDSTATES PATENTS 1,849,703 3/1932 Boller 7139 2,849,280 8/1958 Le Baron eta1. 7139X 2,899,293 8/ 1959 Munckata 7139X 2,942,967 6/1960 Caldwell7139 2,985,527 5/1961 Nossen 71--39 SAMIH N. ZAHARNA, Primary Examinerii A B. H. LEVENSON, Assistant Examiner us. 01. X.R. 23-6 6, 103,1122,7149, 5s, 63

